Quantum Bonding Motion in Ferroelectric PbTiO₃ Perovskite

Abstract

PbTiO₃ perovskite is known as representative ferroelectric as well as BaTiO₃ perovskite. First, we investigate chemical bonding formation related to counter cation in several perovskites. In BaTiO₃ and La_2/3-xLi_3xTiO₃: LLTO perovskites, counter cation forms ionic bonding with constituent atoms. On the other hand, in PbTiO₃ perovskite, covalent bonding is formed between lead cation and constituent atoms. In BaTiO₃ perovskite, the ionic bonding is kept during barium cation migration around tetragonal centre, while in PbTiO₃ perovskite, chemical reaction, in the meaning of partial covalent bonding change, is combined during lead cation migration around tetragonal centre. Based on these scientific knowledge, ferroelectric quantum bonding (QB) motion in PbTiO₃ perovskite is investigated by performing molecular orbital calculation. Stabilisation energy and electrostatic moment (EM) value of PbTi₈O₁₂ unit are larger than BaTi₈O₁₂ unit. Near surface, aperiodic and periodic quantum bonding motions are smoothly connected through the junction. In the application of electric field, though lead cation cannot migrate between neighbouring PbTi₈O₁₂ units, lead cation can migrate near tetragonal centre. Hence, not only ion-conduction but also chemical reaction is combined in ferroelectric QB motion of PbTiO₃ perovskite. In ferroelectric engineering, zirconium doped PbTiO₃ perovskite: PbZr_1-xTi_xO₃ (PZT) perovskite has been employed as ferroelectric material. Finally, we discuss ferroelectricity in PZT perovskite.